Proinflammatory IFNγ Is Produced by but Not Required for the Generation of Eomes+ Thymic Innate CD8 T Cells

Innate CD8 T cells are proinflammatory effector T cells that achieve functional maturation in the thymus prior to their export into and maturation in peripheral tissues. Innate CD8 T cells produce the Th1 cytokine IFN gamma but depend on the Th2 cytokine IL-4 for their generation. Thus, innate CD8 T cells can permute the intrathymic cytokine milieu by consuming a Th2 cytokine but driving a Th1 cytokine response. The cellular source of IL-4 is the NKT2 subset of invariant NKT (iNKT) cells. Consequently, NKT2 deficiency results in the lack of innate CD8 T cells. Whether NKT2 is the only iNKT subset and whether IL-4 is the only cytokine required for innate CD8 T cell generation, however, remains unclear. Here, we employed a mouse model of NKT1 deficiency, which is achieved by overexpression of the cytokine receptor IL-2R beta, and assessed the role of other iNKT subsets and cytokines in innate CD8 T cell differentiation. Because IL-2R beta-transgenic mice failed to generate both NKT1 and innate CD8 T cells, we postulated an in vivo requirement for IFN gamma-producing NKT1 cells for innate CD8 T cell development. In-depth analyses of IL-2R beta-transgenic mice and IFN gamma-deficient mice, however, demonstrated that neither NKT1 nor IFN gamma was required to induce Eomes or to drive innate CD8 T cell generation. Instead, in vivo administration of recombinant IL-4 sufficed to restore the development of innate CD8 T cells in NKT1-deficient mice, affirming that intrathymic IL-4, and not IFN gamma, is the limiting factor and key regulator of innate CD8 T cell generation in the thymus.